This invention relates to electrostatographic imaging systems and more specifically to improved carrier compositions useful in the development of electrophotographic images.
It is well known to form and develop images on the surface of photoconductive materials by electrostatic means, one of the more basic systems being described in C. F. Carlson U.S. Pat. No. 2,297,691. This process is also described in other U.S. patents, including for example, U.S. Pat. No. 2,277,013, U.S. Pat. No. 2,357,809, and U.S. Pat. No. 2,551,582, U.S. Pat. No. 3,220,324 and U.S. Pat. No. 3,220,833. The processes described in these patents generally involve the forming of a latent electrostatic charge image on an insulating electrophotographic element whereby the latent image is made visible by a development step wherein the charged surface of the photoconductive element is brought into contact with a suitable developer mix. As described in U.S. Pat. No. 2,297,691, for example, the resulting electrostatic latent image is developed by depositing on the image a finely-divided electroscopic material referred to in the art as toner. This toner is generally attracted to the areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image and subsequently the toner image can be transferred to a support surface such as paper. This transferred image can then be permanently fixed to the support surface by using a variety of techniques including heat; however, other suitable fixing methods such as solvent or overcoating treatment may be used.
Numerous methods are known for applying the electroscopic particles to the electrostatic latent image including cascade development, touchdown and magnetic brush belt. In cascade development, as described in U.S. Pat. No. 2,618,552, a developer material comprising relatively large carrier particles having finely-divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the electrostatic latent image bearing surface. The composition of the toner particles is selected in order to have a triboelectric polarity opposite to that of the carrier particles. Thus, as the mixture cascades or rolls across the image bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Carrier particles and unused toner particles can then be recycled. This process is fully described by E. N. Wise in U.S. Pat. No. 2,618,552.
In the touchdown process as described in U.S. Pat. Nos. 2,895,847 and 3,245,823, a developer material is carried to a latent image bearing surface by a support layer, such as a web or sheet and is deposited thereon in conformity with the image.
In magnetic brush development, a developer material comprising toner and magnetic carrier particles is carried by a magnet whereby the magnetic field of the magnet causes alignment of the magnetic carriers into engagement with an electrostatic latent image-bearing surface, causing the toner particles to be attracted from the developer to the electrostatic latent image by electrostatic attraction. This process is described more fully in U.S. Pat. No. 2,874,063.
Carrier materials used in the development of electrostatographic images are described in many patents including U.S. Pat. No. 3,590,000, the nature of the material being used being dependent on numerous factors such as the type of development used, the quality of the development desired, the type of photoconductor employed and other factors including durability. Generally, the materials used as carrier surfaces or carrier particles, or coatings thereon, should have a triboelectric value commensurate with the triboelectric value of the toner, in order to enable electrostatic adhesion of the toner to the carrier. Also, the triboelectric charging properties of the carrier should be relatively uniform in order to allow uniform pickup and subsequent deposition of toner. Further, carrier coatings should preferably have a certain hardness primarily for durability purposes but yet made of materials that will not scratch the plate or drum surface upon which the electrostatic image is initially placed. Carriers should also be selected that are not brittle so as to cause flaking of the surface or particle breakup under the forces exerted on the carrier during recycle as such will cause undesirable effects and could, for example, be transferred to the copy surface thereby reducing the quality of the final image. In addition, there are several types of carrier materials which, although having the proper triboelectric properties, are of limited use in a development system because of the limitations they possess, as described above, which result in undesirable results.
Some recent efforts have focused on the carrier particles, and more specifically the coating of these particles in order to obtain a better development system, particularly a developer that can be recycled and does not cause injury to the photoconductor. However, many of the coatings utilized deteriorate rapidly, particularly when used in a continuous process, and sometimes the entire coating separates from the carrier core in the form of chips or flakes which may be caused by poorly adhering coating material that fails upon impact and abrasive contact with machine parts and other carrier particles. Generally, such coated carrier particles cannot be reclaimed and reused, and further, poor print quality results when damaged carriers are not replaced. Also to be taken into consideration is the triboelectric and flow characteristics of coated carriers since such properties may be adversely affected when relative humidity is high. Thus, for example, the triboelectric values of some carrier coatings fluctuate when changes in relative humidity occur and such carriers are not desirable for use in electrostatic systems since they adversely affect the quality of the resulting image.
The importance of carrier coatings takes on increased emphasis in different development techniques. Generally, in order to develop a latent image comprised of negative electrostatic charges, an electroscopic powder and carrier combination is selected in which the powder is triboelectrically positive relative to the granular carrier. To develop a latent image comprised of positive electrostatic charges, such as when employing a selenium photoreceptor, an electroscopic powder and carrier is selected in which the powder is triboelectrically negative relative to the carrier. Thus, where the latent image is formed of negative electrostatic charges, such as when employing organic electrophotosensitive materials as the photoreceptor, it is desirable to develop the latent image with a positively charged electroscopic powder and a negatively charged carrier material.